1. Field of the Invention
This invention relates to epoxy resins and, more particularly, to epoxy resins of improved strength and ductility.
2. Description of the Prior Art
Resins known as epoxy resins have received wide industrial acceptance and are used for many different purposes. The epoxy resins themselves are permanently thermoplastic and require the addition of cross-linking agents or other reactive materials in order that they may be cured to hard, infusible resinous products. However desirable as they may be, the cured epoxy products have a serious disadvantage in that they are inherently brittle. This lack of flexibility has restricted their use in many areas.
It is also known, e.g. in Goldberg et al U.S. Pat. No. 3,388,185 patented June 11, 1968 that the properties of epoxy resins can be improved by blending the resin with a polyfunctional anhydride curing agent.
The Gruenwald U.S. Pat. No. 3,396,145 patented Aug. 6, 1968 also describes curing agents for epoxy resins including long chain polyfunctional aliphatic acids or anhydrides. These are intended for improving the elasticity of the cured resin.
However, the present inventors found that although epoxy resins cross-linked with anhydrides had improved tensile strength, the resins exhibited a brittle fracture with a quite low strain at failure.
The object of the present invention is to provide an improved cross-linked epoxy resin having high tensile strength, high elongation, ductility, high interlaminar shear strength, good processability and low cost.